10 Gigabit Ethernet is an increasingly popular communication standard, with a nominal data rate of 10 Gbit/s. One form of 10 Gigabit Ethernet is IEEE 10 GBASE-T, used to provide 10 gigabit per second connections over unshielded or shielded twisted pair copper wires. The transmit wire-level modulation for 10 GBASE-T is a Tomlinson-Harashima Precoded (THP) version of pulse-amplitude modulation with 16 discrete levels (PAM-16), encoded in a two-dimensional constellation pattern known as 128-DSQ (Double Square QAM).
The receiver circuitry that receives and processes the transmitted 128DSQ symbols includes a slicer circuit that evaluates the received symbols and produces a preliminary decision of each data value. The decision generally involves taking the input data values at the PAM-16 levels, and assigning values based on the proximity of the received value to a particular PAM-16 level. The decision may be done sample by sample in which case the PAM-16 levels are used, or it can be done on pairs of samples in which case the 128-DSQ grid points are used. A forward error correction mechanism (like LDPC coding) may additionally help to make the right decision of mapping the received symbols to constellation points. This decision mechanism, which may include some level of error correction, is referred to as a slicer in this document. A communication system that is designed to meet performance targets in presence of background Gaussian noise may suffer from a burst of correlated noise, particularly in the form of radio frequency interference (RFI).
One proposal in the art to address RFI involves detecting common-mode noise, and utilizing the detected common-mode noise to filter the effects of RFI. In many cases this works well for its intended applications. However, some implementations utilize a fifth channel, with the common-mode representing the 5th channel (in addition to the four data channels). Providing a fifth channel may cause an increase in package pincount for each transceiver chip. Thus, for some applications, it would be desirable to address the RFI problem in a high-speed ethernet system without the need to employ a 5th channel.